It is now generally accepted that fossil fuels are both limited as a resource and cause a net increase in global emissions of carbon dioxide, a “greenhouse gas” implicated in a potential global warming scenario. These fossil fuels, in particular petroleum, are essential for the production of liquid transportation fuels and the vast majority of organic chemicals, in addition to providing a significant proportion of static energy generation.
The only significant alternative source for liquid transportation fuels and organic chemicals is biomass, such as lignocellulosic biomass, and considerable effort has been expended over many decades to produce efficient and economic processes for the conversion of biomass into such fuels and chemicals.
Lignocellulosic or woody biomass is largely composed of hemicellulose, cellulose and lignin. Sources of lignocellulosic biomass include wood and wood residues, agricultural waste such as corn stover, woody grasses, and residential and industrial waste. Each of the main components of lignocellulosic biomass is a valuable material. For example, cellulose is principally comprised of C6 sugars (glucose) which may be further processed for the production of ethanol, a commercial fuel, or recovered as an anhydro-sugar, levoglucosan, or as levulinic acid and fine chemicals, mixed higher alcohols and more valuable fuels. Hemicellulose is comprised of C5 or C6 sugars such as xylose, arabinose, galactose, glucose and mannose. These sugars may be also fermented to ethanol or recovered as furfural and other derivatives and further processed to fine chemicals, alcohols and other commercial fuels. Lignin is a complex polymer which may be further processed to fine chemicals (such as phenol and fuel additives) or may be used as a direct fuel for the generation of heat and power for process and export.
The lignin component of lignocellulosic biomass materials gives physical strength to the biomass, and is tightly bound to the hemicellulose and cellulose components. Therefore, while it is desirable to fractionate the biomass, the presence of the lignin makes fractionation difficult, and the harsh conditions required for fractionation can result in breakdown of the carbohydrates into less desirable products.
Various methods have attempted to remove the carbohydrate sugars present in hemicellulose and cellulose from the biomass. For example, biochemical and chemical processes using enzymes, solvents, acid, alkali, or hot water can be used to attempt to dissolve the carbohydrate or lignin components of the lignocellulosic biomass with or without concomitant hydrolysis. In addition, various forms of pretreatment such as steam explosion, hot water, and acid or alkali processes, attempt to make the carbohydrates accessible for separation. However, separating the biomass into fractions and isolating each of these fractions, while avoiding the production of byproducts and minimizing the consumption of energy (and therefore production cost) remains difficult.
The processes discovered to date for the conversion of biomass into fuel can be generally considered to be included in one or other of the following two categories. One category is a thermochemical treatment of whole biomass, without fractionation or separation of the component parts of the biomass, by means of pyrolysis, gasification or liquefaction, generating primarily a crude bio-oil or synthesis gas mixture. The other category includes physical and chemical pre-treatments of whole biomass, aimed at destruction or neutralization (rather than separation and collection) of the volatile or extractable components and the hemicellulosic components of the biomass (which would otherwise inhibit the subsequent conversion step or steps), followed by a chemical or microbiological (enzymatic) hydrolysis of the cellulosic components and a microbiological fermentation of the resultant cellulosic sugars. Other processes are also known which are generally of a chemical nature and carried out in the liquid phase, such as solvent dissolution and separation of one or more of the major components, including supercritical extraction processes. All such processes are generally directed at liquid transportation fuel production or at production of a specific chemical or limited range of chemicals or of products such as fiberboard.